This invention is concerned with a process for removing floating oil-based materials from the surface of water contained in a tank and retaining said oily materials on the surfaces of the tank as the water is discharged. The invention is particularly concerned with the removal of oil-based materials from the surface of water contained in the steel tanks of cargo carriers such as tankers.
The present ballasting operations for crude oil tankers between the discharge port and the loading port comprise a three-step process. After oil cargo discharge in the discharge port, the tanker makes up its required departure ballast by taking on water ballast in addition to its segregated ballast. Usually this ballast is pumped to several wing tanks of the tanker and is designated as dirty ballast. At sea, enroute to the loading port, another set of tanks, usually two or three center tanks, is first washed and cleaned and then loaded with sea water. Such ballast is designated as clean ballast. This clean ballast contains substantially no oil within the water but may have a small amount of oil floating on its surface and is suitable for discharge at the loading port on arrival provided a substantial portion of the water is retained to avoid discharging the floating oil residues. Finally, the dirty ballast is discharged at sea following established procedures to prevent pollution and is essentially exchanged for the clean ballast. Such operations involve handling twice the amount of additional ballast required for the ballast passage and the time, cost and effort in handling such ballasting operations is considerable. For example, in the case of very large cargo carriers (VLCC), up to 60,000 tons of ballast, in addition to the segregated ballast, is required during a normal ballast passage. This means that another 60,000 tons of dirty ballast must be processed at sea, which processing involves substantial fuel consumption and man-hours.
One solution to the problem of first loading dirty ballast then clean ballast, and then unloading the dirty ballast would be to load clean ballast directly into tanks that have been properly cleaned at the discharge port either by crude oil washing only (COW); crude oil washing and water rinsing, or crude oil washing and water washing. A stripping operation is usually applied after cleaning to remove the oil heel left in the bottom of the tank. After directly loading the ballast at the discharge port any residual oil and possibly portions of the sludge remaining in the tanks floats on the surface of the ballast water.
To discharge direct-loaded clean ballast without threat of oil pollution, the residual oil and sludge floating on the ballast surface must either be removed from or retained in the ballast tank. Attempts have been made to develop various methods of mechanical skimming of the ballast surface, examples of which include moving beds of plastic foam across the ballast surface; using the ship's rolling motions, streams of air or water, or rising bubbles to drive oil over fixed and adjustable weirs built into a tank bulkhead; or moving fixed or tethered floating skimming units over the ballast surface. Such mechanical skimming methods have generally been unsatisfactory for a variety of reasons including the interference of the ship's motion, inability to effectively control the particular method and fouling of moving parts by accumulated sludge.
The field of surface films and spreading has been dealt with extensively in the art. Exemplary of this art is the treatment of this topic by William D. Harkins, entitled "The Physical Chemistry of Surface Films", Reinhold Publishing Corporation, 1952. Other reference material dealing with spreading of oils on water, spreading pressure and surface collecting agents include the following: (1) The Journal of Chemical Physics, Vol. 9, No. 10, pp. 729-741, October 1941; (2) Journal of Colloid and Interface Science, Vol. 28, No. 1, pp. 106-117, September 1968; and (3) "Surface Collecting Agents", Shell Oil Company, March 1978.
The present invention, on the other hand, provides a method for the chemical skimming of floating oil-based materials such as crude oil and sludge from the surface of water contained in a tank and retaining the oily materials on the surfaces of the tank as the water is discharged. The method is particularly useful for removing residual crude oil and sludge from the cleaned ballast tanks of tankers which have been directly loaded with ballast at the cargo discharge port following crude oil washing. It has been demonstrated in shipboard trials using the present method that the oil content in the discharged water was less than 15 ppm. even at low tank innages which content meets the proposed regulations of the Intergovenmental Consultative Organization (IMCO). It has also been demonstrated that the method is largely independent of the ship's motion, can be easily controlled, and is economical since it eliminates the fuel consumption attributable to ballast handling operations at sea. Other advantages of the method include reduction of the amount of water required for tank washings prior to taking on clean ballast, the elimination of dirty ballast altogether if tank cleaning and stripping operations are employed at the discharge port, and added protection against oil discharge in the event of poor tank washing for existing ballast handling procedures or failure to retain sufficient clean ballast to prevent the discharge of floating oil or sludge residues. In the case of tanks which have been crude oil washed only and then directly loaded at the discharge port, the need for water rinsing and other procedures for disposing of dirty ballast at sea may be substantially eliminated.